Peristaltic pumps are preferred for many procedures because they can pump fluid through tubing without exposing the fluid to contact with the tubing exterior or any of the pump components. This feature is particularly desirable in medical and laboratory procedures where maintaining the sterility of a fluid is often vital. A problem arises, however, where peristaltic pumps are used with biological fluids. Various biological fluids are damaged by excessive pressure. For example, placing blood under high pressure in an extracorporeal tubing system may result in the blood cells being crushed.
A peristaltic pump is a volumetric positive displacement pump that moves fluid through a tube by progressively compressing the fluid tube in one direction. A peristaltic pump typically comprises a housing having a semi-circular internal raceway for receiving a fluid tube and a rotating member mounted in the center of the semi-circle formed by the raceway. The rotating member generally has roller elements that compress the fluid tube against the raceway. When the roller elements compress the tube, they also exert pressure on the internal fluid. It is, therefore, desirable to set the roller elements, of the rotating member, to a position predetermined to efficiently move the fluid through the tube without damaging the fluid. It is important that any method used to ascertain this position not compromise the sterility of the tubing fluid.
A common method currently used to set degrees of occlusion in peristaltic pumps involves measuring the drop rate of a column of fluid through a tubing loop. The rate at which the fluid drops is proportional to the extent to which the tubing is occluded. The pump occlusion mechanism is then manually adjusted to achieve a specified drop rate which correlates to the desired degree of occlusion. The fluid used to determine the drop rate, and sometimes the tubing as well, is discarded. This method is time consuming and may require using tubing in addition to that already required for the procedure.
Furthermore, this drop rate method can only be carried out before the pump is employed in a procedure. Often the flexible tubing used with peristaltic pumps distorts or relaxes during a procedure. As a result, the degree of occlusion in the tubing of the preset system will also change. The current drop rate method does not allow occlusion degree variations to be monitored during the course of a procedure.
Other methods are known for measuring degrees of occlusion in peristaltic tubing during a course of a procedure. These methods typically include an occlusion indicator or an occlusion alarm that activates when a preset threshold is exceeded. These methods, however, do not allow for adjustment of the degree of occlusion once it has been detected.
Prior art attempts to improve occlusion detection in peristaltic pumps have included:
U.S. Pat. No. 5,103,211 (1992) to Daoud et al. discloses an apparatus for detecting pressure and occlusion in a fluid line. The disclosed fluid line is driven by a peristaltic pump. The rotating member of the pump has a plurality of fingers for exerting pressure on the line. One of the fingers is a sensor follower finger. A strain gauge, mounted on the sensor follower finger, generates a signal indicating the amount of force that the finger is exerting on the line. A signal processor, also mounted on the sensor follower finger, receives the signal and sounds an alarm if an occlusion is determined.
U.S. Pat. No. 5,049,047 (1991) to Polaschegg et al. discloses a peristaltic infusion pump with means for measuring the internal diameter of the associated fluid tube. Fluid infusion rate, in such a system, is dependent on the internal diameter of the pump tube. The Polaschegg ('047) invention uses either a mechanical compression system or an ultrasound system to measure the pump's internal tube diameter and adjust infusion rate accordingly.
U.S. Pat. No. 4,836,752 (1989) to Burkett discloses a device for detecting partial restrictions in an IV fluid line, where the fluid is driven by a peristaltic pump. The device comprises a gauge for detecting dimensional variations in the outside diameter of the fluid line. The device then correlates the dimensional variations with changes in the fluid pressure in the line. The device sounds an alarm when it detects a pressure corresponding to a preset threshold pressure.
U.S. Pat. No. 4,373,525 (1983) to Kobayashi discloses an apparatus and method for detecting occlusion in the fluid infusion tube of a peristaltic fluid-infusion pump. The Kobayashi ('525) invention detects changes in the distance between opposing walls of the fluid infusion tube, or in other words, the tube diameter. A change in the distance between the walls of fluid infusion tube reflects a change in the internal pressure. An increase in the fluid infusion tube's internal pressure correlates to an occlusion. One disclosed embodiment uses a bridge circuit, a sampling circuit, and a comparator to detect this change in infusion tube diameter.
It is known that the electrical conductance of a fluid in a tubing loop, comprising a closed electrical circuit of known cross sectional area and length, may be measured by inducing and sensing an alternating electrical current in the fluid. The magnitude of the induced current is proportional to the conductance of the fluid. A cell for measuring the conductance of a biological fluid without contacting the fluid in a closed non-metallic conduit is disclosed in U.S. Pat. No. 4,740,755 entitled "Remote Conductivity Sensor Having Transformer Coupling In A Fluid Flow Path," issued Apr. 26, 1988 to Ogawa and assigned to the assignee of the present invention, the disclosure of which is hereby incorporated, in its entirety, by reference.
At present, adjustment of occlusion amounts in peristaltic pumps is generally achieved manually or maintained through spring loaded roller elements. Present manual adjustment is time consuming and may only be carried out before a procedure is started. This is problematic because, as discussed above, tubing often relaxes or distorts during a procedure which requires that the roller position be adjusted to maintain the desired degree of occlusion.
Spring loaded rollers are also problematic. They tend to fully occlude the tubing which may excessively damage a biological fluid contained in the tube. This is a particularly serious problem during heart by-pass operations where a patient's blood is repeatedly circulated through a peristaltic pump.
The following exemplify prior art occlusion adjustment devices in peristaltic pumps:
U.S. Pat. No. 4,548,553 (1985) to Ferster discloses a peristaltic pump improvement that comprises a mechanism for externally controlling the position of the pump's rollers, thereby setting the pressure exerted by the roller on the tubing fluid to a desired level. The Ferster ('553) invention comprises two roller elements, positioned back-to-back and biased towards each other by a spring. The tip of an inverted cone is disposed between the roller element backs. The cone may directed downward manually by turning an adjusting screw. As the cone is adjusted downward, the roller elements are further biased outward, exerting increased pressure on the tubing and its fluid.
U.S. Pat. No. 3,463,092 (1969) to Meyer discloses a pump having a tube mounted around rotatable member. The tube is mounted under tension, thereby obviating the need for a raceway or internal arcuate wall. The rotating member comprises a number of rollers disposed in a circle around a inverted conical nut. The rollers may be balls.
As the nut is displaced downward the balls are radially biased outward, thereby exerting increased pressure on the tube and its internal fluid.
U.S. Pat. No. 3,955,902 (1976) to Kyvsgaard, U.S. Pat. No. 4,174,193 (1979) to Sakakibara, and U.S. Pat. No. 4,522,571 (1985) to Little all disclose peristaltic pumps having raceways with angled interior walls and correspondingly angled roller elements. All of the above inventions comprise means for manually adjusting the size of the gap between angled interior walls of the raceway and the roller elements, thereby adjusting the pressure exerted on the associated tubing and its fluid.
U.S. Pat. No. 4,568,255 (1986) to Lavender et al. discloses a peristaltic pump having a single cam, mounted between rotor and pump arms. The position of both pump arms may be simultaneously adjusted by manually rotating the cam via an attached knob.
U.S. Pat. No. 3,885,894 (1975) to Sikes discloses a peristaltic pump in which the roller elements are spring biased toward the raceway walls. The roller elements are, simultaneously, limited with respect to how close they may approach the race way walls. An adjustable stop sets a predetermined minimum gap between the race way walls and the roller elements. The Sikes ('894) invention allow the roller elements to self-adjust, to the extent of the preset minimum gap, to tubing variations.
Further prior art patents of interest that teach various means of adjusting roller position and, therefore, tubing occlusion in peristaltic pumps include: U.S. Pat. No. 315,667 (1885) to Serdinko in which roller position is adjusted by thumb screws. U.S. Pat. No. 460,944 (1891) to Burson in which roller position is adjusted via a gear wheel. U.S. Pat. No. 487,136 (1892) to Truax in which roller position is adjusted by turning a disc which engages the roller arms. U.S. Pat. No. 3,079,868 (1963) to Ormsby in which roller position may be adjusted by turning a screw which then engages a compression spring which then directs the roller towards the race way wall. U.S. Pat. No. 3,787,148 (1974) to Kopf in which rollers are drawn inward away from the tubing, when a slot cam is rotated clockwise.
The following prior art peristaltic pump improvements may also be of interest:
U.S. Pat. No. 5,052,900 (1991) to Austin discloses a pressure relief valve for positive pressure pumps, such as peristaltic pumps. The valve comprises a piece of bypass tubing connecting the outlet end of a tubing loop to the inlet end. A pressure limiting device is centered on the bypass tubing. The pressure limiting means consists of two bars placed on opposing sides of the bypass tubing and connected at their ends by elastic bands. When the bypass tubing contains enough fluid to exert sufficient pressure, the bars are displaced away from each other allowing the fluid to flow from the outlet to the inlet region of the tubing.
U.S. Pat. No. 4,650,471 (1987) to Tamari discloses a flow regulating device for a peristaltic pump that comprises an outer tube that surrounds the inner flexible fluid containing tube, thereby forming a chamber. The outer tube contains at least two access ports communicating with the inner chamber. Pressure gauges, occlusion regulating devices, alarm systems and other such devices can be attached to the access ports.
The present invention offers many advantages over the prior art. Many prior art occlusion detectors may only detect and adjust occlusion at the outset of a procedure, but not while the peristaltic pump is in motion. This is a problem because often during the course of a procedure the shape of the tubing distorts; therefore, a degree of occlusion set at the outset of a procedure will under or over occlude the distorted tubing during the course of a procedure. Additionally, the degree of occlusion may not be changed at will in prior art pumps during the course of a procedure.
Furthermore, many prior art pumps require lengthy set up times. In prior art pumps lacking retractable rollers, it is difficult to load the tubing into the pump raceway during pump set up. Additionally, many prior art pumps require extra tubing to set the desired degree of occlusion at the outset of the procedure.